1. Field of the Invention
The present invention relates to a hemming apparatus for a double plate structure consisting of an outer plate and an inner plate and more particularly to a hemming apparatus for bending an edge portion of the outer plate so as to be folded around the opposed edge portion of the inner plate.
2. Description of the Prior Art
A bonnet, side doors, a luggage compartment door, etc. of a motorcar are usually of a double structure consisting of inner and outer plates, in which structure the edge portion of the outer plate is bent and folded around the opposed edge portion of the inner plate.
Taking a bonnet of a motorcar as an example, an operation of bending an outer plate by a conventional hemming apparatus will be described.
First, an outer plate 2 of a work is placed on a bottom form 1 of a press as shown in FIG. 1, and an inner plate 3 is then set on the outer plate 2.
A top form 4 of the press is lowered to allow a preliminary bending cam 5 attached thereto to come into engagement with a preliminary bending arm 6 pivotally connected to the bottom form 1. The preliminary bending arm 6 is then rotated toward the work to allow a preliminary bending blade 7 attached to the arm 6 to press against an edge portion of the outer plate 2. Thus, the edge portion of the outer plate 2 is bent to such an extent that it is conveniently subject to a primary bending operation which is carried out afterwards.
When the top form 4 is then further lowered, the preliminary bending cam 5 comes out of engagement with the preliminary bending arm 6 as shown in FIG. 3, so that the arm 6 is returned to its original position to thereby render the space above the bent edge portion of the outer plate 2 clear.
When the top form 4 is lowered still further, a laterally slidable slider 8 provided on that portion of the top form 4 which is arranged to be above the bent edge portion of the outer plate 2 is moved close thereto as shown in FIG. 4, so that a primary bending blade 9 attached to the slider 8 comes into engagement with the primarily-bent edge portion of the outer plate 2. Thus, the edge portion of the outer plate 2 is subjected to a finishing bending operation so as to be bent around the opposed edge portion of the inner plate 3.
In this case, it is necessary that at the same time as the primary bending blade 9 moves downwardly, it should slide laterally in the direction in which the work is pressed. Accordingly, slider 8 is adapted to move laterally as it moves downwardly, by providing a slide cam 10 thereon which is engageable with a cam 11 having an inclined surface and attached to the bottom form 1.
A conventional hemming apparatus mentioned above permits bending work up to a final stage by conducting preliminary and primary bending operations independently in the mentioned order during one downward movement of the top form 4 for each of the bending operations.
However, the preliminary bending mechanism of this type should be removed from the space above the work before starting a primary bending operation. Accordingly, this type of hemming apparatus necessarily has a complicated construction. Moreover, since the shapes of the peripheral edges of the work are often different on all sides of a bonnet, the work may possibly be displaced on the bottom form 1 during a preliminary or primary bending operation and as a result, the work may not be properly formed. Furthermore, a large downward force is applied to the outer plate 2 at the primary bending so that such a portion thereof that is designated by a symbol B in FIG. 4 buckles, for example in a shape of creases, wrinkles or the like.
Therefore there is a need for a hemming apparatus of this kind to be further improved so as to solve the above-mentioned problems.